Stabilized low pressure triode sputtering apparatus



April 1, 1969 ZENJIRO'ODA ET A| 3,436,332

STABILIZED LOW PRESSURE TRIODE SPUTTERING APPARATUS Filed July 12, 1966 Sheet of 2 L j 24 '.60 L L I' 4 1 2 &

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JIIIJIIIIIJIJJ ATTORNEYS April 1, 1969 ZENJ|RO QDA ET AL 3,436,332

STABILIZED LOW PRESSURE TRIODE SPUTTERING APPARATUS Filed July 12, 1966 Sheet 3 of 2 ATTORNEYS United States Patent 3,436,332 STABILIZED LOW PRESSURE TRIODE SPUTTERING APPARATUS Zenjiro Oda, Koichi Saito, Tatsuo Asamaki, and Tokio Mori, Tokyo, Japan, assignors to Nippon Electric Company Limited, Tokyo, Japan, a corporation of Japan Filed July 12, 1966, Ser. No. 564,698 Claims priority, application Japan, July 15, 1965, 40/42,815 Int. Cl. C23c 15/00 U.S. Cl. 204-298 3 Claims This invention relates to a sputtering apparatus for forming a thin film of metal or other materials.

By means of sputtering apparatus, it is possible to provide a film of uniform thickness, and the same is very suitable for forming a thin film of wide dimensions because a plane sputtering source may be employed. Furthermore, a film of a refractory material, such as tantalum or the like, can be formed with such apparatus, and therefore it has come to be widely used in the manufacture of micro-electronic circuits.

In a sputtering device of the type referred to, it is desirable for a number of reasons that the sputtering operation be conducted at a comparatively low gas pressure. However, in a conventional type device, especially in the three-element type sputtering device, which comprises a cathode, an anode and a target, once the electric-gas discharge is quenched when the gas pressure drops below 1 l0- torr, it will not refire unless supplied with gas at a considerably higher pressure generally in the range of 100 times greater than the discharge quenching pressure. Consequently, discharge under such a low pressure becomes unstable, and the sputtering operation also becomes unsteady, resulting in a low operating efiiciency for the device.

Accordingly, it is an object of the present invention to provide a novel sputtering apparatus having a steady operation and high efficiency by stabilizing the discharge at low pressures.

It is another object of this invention to provide a sputtering apparatus wherein the useful life of the filament or thermionic cathode employed is increased.

All of the objects, features and advantages of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of the invention taken in conjunction with the accompanying drawing, in which:

FIG. 1 shows a vertical sectional view of one embodiment of the invention;

FIG. 2 shows a section along the line 2-2 of FIG. 1;

FIG. 3 shows an enlarged view of portions of the thermionic cathode and the starting electrode seen in FIG. 1; and

FIG. 4 shows the target current versus pressure characteristic of the embodiment described compared with that of a conventional sputtering apparatus.

According to the sputtering apparatus of this invention, a starting electrode is disposed adjacent to a thermionic cathode or filament, to which is applied a relatively high positive electric potential. An extremely stable discharge characteristic can then be obtained in the region of the critical pressure of the sealed gas because of the positive potential applied to the starting electrode. Provision is also made to prevent damage to the filament or cathode by bombardment from the positive ions in the plasma produced by discharge, increasing the filament or cathode life.

Referring now to FIGS. 1, 2 and 3, there is shown an electron emitter structure according to the invention. This structure 10 in FIG. 1 comprises a tungsten filament 11 as an electron source with lead-in wires 12 therefor, 21 starting electrode 13 Which is one of the features of this invention, a lead-in wire 14 for the starting electrode 13, a glass insulating tube 15 covering the wire 14, a guide tube 16 for introducing plasma to an anode 31, a water cooling tubulation 17 in the form of a coil embracing the tungsten filament 11, an insulated terminal 18, a shield 19 to protect the filament from being bombarded with ions and also to prevent the filament material from sputtering onto the anode, an electric power supply 21 for the filament, an electric power supply 22 for the starting electrode, a stabilizing resistor 23, and an electric power supply 24 for biasing the filament to prevent the electrons from entering into the guide tube 16.

The device further comprises an anode structure 30 for forming a plasma around a target 41 by firing an electric discharge between the filament 11 and the anode 31. This structure includes the anode 31, a member 32 attaching this anode to its lead-in wire 33, a glass insulating tube 34 covering the wire 33, an insulated terminal 35, and an anode electric power supply 36. A target structure 40 is also provided and comprises the target 41 which is a sputtering source, a connecting member 42 for connecting the target to its lead-in wire 43, a glass tube 44, an insulated terminal 45, and a target electric power supply 46.

A substrate structure 50 is provided on which a film is formed by depositing the metal atoms sputtered from the target 41. This structure comprises a substrate 51, a connecting member 52 for connecting the substrate to its lead-in wire 53, a glass tube 54, an insulated terminal 55, and a substrate bias electric power supply 56. A magnetic field supplying structure 60 is provided for forming the plasma more effectively and comprises a coil 61 and its electric power supply 62. The structures described above are sealed in a vacuum envelope 70'. This vacuum envelope 70 comprises a bell-jar 71, a gasket 72, a vacuum vessel 73 having a connecting tube 74 for connection to the usual vacuum pump and the gas introduction system, and a base plate 75.

The apparatus emlbodying this invention is operated in the manner now to be described. Inasmuch as a sputtered film is adversely affected by residual active gases, such as oxygen, the apparatus is exhausted to a pressure below l0 torr, and the electrodes within the apparatus are degassed by bakeout, electron bombardment or other known methods. An inert gas, such as argon, or a special gas selected to provide a desired film, is then injected into the envelope and the inflow of said gas and the pumping speed are adjusted so that the interior of the apparatus will be maintained at a proper pressure (usually around 1 10- torr). These processes as described above are similar to those used With conventional sputtering devices.

Next, the voltage of the anode electric power supply 36 is set at approximately 60400 v. DC, the voltage of the filament electric power supply 21 is set at approximately 7 v. AC or DC, and a magnetic field of approximately gausses is supplied by the coil 61. Under these conditions, no electric discharge occurs. However, when the starting electrode 13 is energized by connecting the same to a potential of 200500 v. DC on the electric power supply 22 through a stabilizing resistor 23 of 0.7-20KQ, an electric discharge occurs between the filament 11 and the anode 31. Under these conditions, the anode current lbecomes several amperes and a plasma is formed in the region of the target 41. It should be noted that a reduction in the value of the resistor 23 reduces the resulting anode current.) Then, by setting the voltage of the target power supply 46 at a negative potential in the range of 0.3-3 kv., the target 41 becomes negative and pulls only positive ions out of the plasma. At this time, the target current is about 20-200 ma. The target 41 is subjected to ion-bombardment and the target material is sputtered and deposited on the substrate 51 to form a thin film thereon. Although a small portion of the positive ions are attracted to the cathode or filament these ions are prevented from bombarding the filament -11 by means of the positive electric potential of the starting electrode 13 that is disposed adjacent to the filament, thus protecting the filament from damage.

The substrate 51 is usually grounded, however, if necessary, it is possible to employ a positive or negative DC electric source or an AC electric source as the substrate bias electric power supply 56. The bias electric power supply 24 is not ordinarily necessary, however, it is used in order to prevent the electrons emitted from the filament 11 from entering into the tube 16, a bias of approximately -20 v. positive potential being preferred.

Referring now to FIG. 4, there is shown a graph wherein the pressure P is plotted in logarithmic scale on the abscissa, while the target current I is plotted on an arbitrary scale on the ordinate. It will be seen that the target current I decreases along the curve a when decreasing the pressure in the bell-jar 71, While maintaining the anode voltage constant, and that the electric discharge ceases at a pressure P At this pressure, the sputtering also stops. In order to fire the electric discharge again, it is necessary in the conventional type device to raise the pressure to the value P which is much higher than the electric discharge quenching pressure P (the target current characteristic follows the path F-A-E-F). By contrast, in the sputtering apparatus according to the present invention wherein the starting electrode -13 is provided, the discharge is started at a pressure P which is slightly higher than the electric discharge quenching pressure P and the target current is increased along the path B-F.

Accordingly, with the sputtering device embodying the present invention, it is possible to perform a steady sputtering operation, and furthermore, to :do so at a lower pressure than the discharge starting pressure P of the conventional device which is not provided with a starting electrode. If the electric potential of the starting electrode 13 is made the same as that of the anode 31 by using the same electric power supply for the starting electrode and the anode and by excluding the stabilizing resistor 23, the discharge will then be started at a pressure P In this case, a relatively large electric current will flow in the starting electrode 13, due to the elimination of the stabilizing resistor, and consequently, the target current will be decreased to some extent compared with the case where the stabilizing resistor is employed. Under these conditions, the current follows the curb b, along the path G-A-C-G. If the electric potential of the starting electrode 13 is lowered to less than the anode electric potential, by means of the stabilizing resistor 23, the electric discharge is started at a pressure P In this case, the target electric current is varied along the curve a, and follows the path F-A-DF. In each of the two cases immediately above, it will be seen that it is still possible to effect a steady sputtering operation at a lower pressure than the discharge starting pressure of conventional devices.

The following experimental results were obtained when the anode voltage was 65 v., the electric source voltage for the starting electrode was 200 v. and the target voltage was -500 v:

In the above, a 2K9 stabilizing resistor 23 was used and the electric current flowing into this resistor became substantially zero upon discharge and was 20 ma. at the time of cessation thereof, and consequently, the electric voltage on the starting electrode was actually v.

The significant advantages of the sputtering apparatus according to this invention are that:

(1) The sputtered film will be substantially unaffected by residual gases while a steady and high efiiciency sputtering operation is achieved at a low pressure.

(2) The life of the filament will be substantially prolonged by preventing damage to the filament ordinarily resulting from bombardment of positive ions in conventional sputtering devices.

Although this invention has been described with respect to a particular embodiment thereof, it is not to be so limited as various changes and modifications may be made therein. For instance, in a similar device, we found that the discharge quenching pressure P was raised by shortening the distance between the filament and the anode. Accordingly, if the distance between the filament and the anode is elongated, for instance, by means of a zig-zag path as one example, the discharge quenching pressure P can be lowered from the particular example described. Concerning the location of the starting electrode, any region within the discharge path between the anode and the filament is suitable, so long as a clear path is provided between these elements. Good results are obtained by positioning the starting electrode about 10 cm. in front of the filament. Further, regarding the electron source, any filament or cathode can be used so long as it emits electrons. Also, the starting electrode may be of various configurations, such as a coil, mesh, rod or wire shape. The tube 16 is bent into an L-shape to prevent the sputtered cathode material from depositing on the target and the substrate, and to reduce the ionbombardment to the filament as much as possible. Furthermore, in order to guard against contamination of the insulating tubes 34, 44 and 54 from the sputtered material, another set of glass tubes may be employed to surround the same, and the outer contaminated ones may be replaced as necessary. The electric power supply 22 for the starting electrode need not necessarily be a DC electric source, but may be AC as well. Various modifications are also possible as to the arrangement of the electrodes and their relative positions. It is essential, however, that the starting electrode be positioned adjacent to the electron emitting source.

While the foregoing description sets forth the principles of the invention in connection with the specific apparatus and modifications thereof, it is to be understood that the description is made only by way of example and not as a limitation of the scope of the invention as set forth in the objects thereof and in the accompanying claims, since there are various other constructions suitable for carrying out the purposes of this invention.

What is claimed is:

1. In a cathode sputtering apparatus comprising: an enclosure; means for evacuating said enclosure and providing an ionizable atmosphere therein; an ion target of the material to be sputtered mounted in the enclosure; means for mounting a substrate in the enclosure with the surface of the substrate extending substantially parallel and spaced from said target; means in said enclosure for sustaining an electrical discharge between and generally axially parallel to the surface of said target and said substrate, said means including an anode and a thermionic cathode for emitting electrons, said anode and said cathode being separate and independent from said substrate and said target; the improvement wherein said means for sustaining said discharge includes a positively biased starting electrode disposed in immediately adjacent spaced relationship with said thermionic cathode and in the path of said electrons, whereby a steady sputtering operation is provided at reduced gas pressure within the envelope.

2. The apparatus of claim 1 which further includes a shield provided between said thermionic cathode and said 5 6 anode to prevent the migration of material from said References Cited thermionic cathode to said anode and to protect said thermionic cathode from the bombardment of ions, Where- UNITED STATES PATENTS by the life of said thermionic cathode is prolonged. 3,305,473 2/1967 Moseson 204-298 3. The apparatus of claim 1 which further includes a source of potential for positively biasing the biased elec- 5 ROBERT MIHALEK Prlmary Exammer' trode and a stabilizing resistor connected in series between US. Cl. X.R. the starting electrode and the source of potential for controlling the starting of said sputtering. 204192 313 197 207 

1. IN A CATHODE SPUTTERING APPARATUS COMPRISING: AN ENCLOSURE; MEANS FOR EVACUATING SAID ENCLOSURE AND PROVIDING AN IONIZABLE ATMOSPHERE THEREIN; AN ION TARGET OF THE MATERIAL TO BE SPUTTERED MOUNTED IN THE ENCLOSURE; MEANS FOR MOUNTING A SUBSTRATE IN THE ENCLOSURE WITH THE SURFACE OF THE SUBSTRATE EXTENDING SUBSTANTIALLY PARALLEL AND SPACED FROM SAID TARGET; MEANS IN SAID ENCLOSURE FOR SUSTAINING AN ELECTRICAL DISCHARGE BETWEEN AND GENERALLY AXIALLY PARALLEL TO THE SURFACE OF SAID TARGET AND SAID SUBSTRATE, SAID MEANS INCLUDING AN ANODE AND A THERMIONIC 